While access has been restored, the bank has limited access due to the large numbers of users expected to try to log on following the outage.

Online chaos

Regulators are investigating the incident, which was caused by a system switchover finalising TSB’s separation from its former owner, Lloyds Banking Group.

Following the changeover, customers were left unable to view their balances, and some were given access to other people’s accounts.

TSB took its mobile app and online banking offline “for a few hours” on Tuesday following a customer backlash from the problems.

It had hoped to restore service on Tuesday afternoon, but was unable to do so because of excessively high demand.

On Wednesday some users said the system was now so slow it was nearly unusable.

A small business told the BBC it had taken an hour and a half to make one payment online, making it difficult to pay wages or suppliers.

Regulatory response

Nicky Morgan MP, chair of the Treasury select committee, said in a letter to Pester: “It simply isn’t good enough to expose customers to IT failures, including delays in paying bills and an inability to access their own money.”

She asked the bank to explain how it would compensate customers affected by a breach of potentially sensitive personal information.

The Financial Conduct Authority said it was working with TSB “to ensure customers are properly communicated with and are not left out of pocket”.

“We will be talking to the firm to understand exactly what went wrong and the steps that they are taking to ensure something like this does not happen again,” the regulator said.